Curved Earth - Sun rays hit Earth at angles. Since the surface of the Earth is curved, rays from the Sun strike the Earth’s surface at different angles. Areas with more direct sunlight (angles close to 90°) have warmer

temperatures; areas with less direct sunlight have cooler temperatures. The uneven heating of the earth’s surface is what causes weather.

Warmer temperatures cause the air to rise and expand.

• the volume increases

• the density decreases

Cooler temperatures cause the air to contract and sink.

• the volume decreases

• the density increases

Air moves from areas of higher density (or pressure) to lower density. This movement causes wind.

A high pressure system is more dense than the air around it, so it descends to the Earth’s surface. This air is cool and dry with skies that are usually clear and sunny.

A low pressure system is less dense than the air around it, so it rises. As it rises, it cools and water vapor in the air condenses forming clouds and possibly precipitation.

There are two sets of regulations that govern how pilots operate depending on visibility conditions:

Visual Flight Rules (VFR) are the regulations pilots may follow when visibility is at least three miles (pilots can see things three miles away) and the ceiling (the lowest level of clouds covering at least half the sky) is at least 1000 feet above the surface of the Earth.

Instrument Flight Rules (IFR) are the regulations that pilots who are instrument-rated must follow when visibility is less than three miles or the ceiling is below 1000 feet above the surface of the Earth.

Turbulence and wind shear

Wind can move at various speeds and in various directions. Wind affects the movement of the aircraft. Updrafts and downdrafts move vertically to the aircraft. Headwinds and tailwinds move horizontally and parallel to the aircraft; crosswinds move horizontally and perpendicular to the aircraft. When wind changes direction or speed over a short distance, it is called wind shear.

Icing

Condensation or precipitation in the air can accumulate on aircraft in the form of ice. This ice can affect aircraft performance in several ways. Ice blocking engine air intakes reduces the engine’s power. Ice forming on the wings decreases lift. Ice forming on control surfaces compromises the ability to control the aircraft. Ice forming on aircraft increases the weight of the aircraft, but more importantly, it increases aerodynamic drag at the same time that available power to overcome it may be reduced.

Microbursts

A windblast from a thunderstorm less than 2.5 miles in diameter is a microburst. Microburst winds are very fast and can create a ring of wind. Aircraft flying into this ring experience sudden shifts in wind. A headwind can switch to a tailwind causing the aircraft to lose lift and descend. Microbursts are very dangerous.